Disclosed in European Offenlegungsschrift 0 247 767 is a controllable boring machine for drilling holes in the ground, in particular for laying supply lines such as water pipes or cables beneath streets or embankments or other building structures and obstacles without digging trenches, that consists essentially of a pneumatic or hydraulic drive unit arranged on a mounting which statically pushes a drill rod further and further into the ground and sets it in rotation. At the tip of the drill rod there is a ram boring machine having a percussion piston which can reciprocate in a tubular housing of the machine. The ram boring machine has a head with an oblique face by which the ram boring machine and the drill rod that is pulled along and/or pushed in behind it is steered along a certain radius from the depth of the earth bore made in the ground up to the surface. However, because the ram boring machine and striking tool are arranged ahead of the drill rod it is not possible to prevent the machine, particularly in the case of soft soils, from deviating downwards from the running direction due to its weight, so that frequent correction of the direction is required.
The majority of compressed-air-driven ram boring machines connected at the rear end via supply hoses to a compressed-air source arranged outside of the earth bore are turned on and off by normal shut-off valves, e.g. ball valves. As the length of the hose between the shut-off valve at the compressed-air source and the ram boring machine may be some 20 to 100 m, the pressure in the hose and in the ram boring machine builds up relatively slowly after the shut-off valve is opened. As this occurs the percussion piston can be displaced unfavourably in the ram boring machine so that the front and rear faces of the percussion piston are acted on by the same pressure, so that a dead center position results from which the percussion piston can only be started up by a pulse of compressed air. Furthermore the length of the air column can lead to uncontrollable compression effects, and possibly to self-oscillations of the air column, which make starting up the machine difficult. Moreover the ram boring machine often becomes iced up due to the expansion of the compressed air, which is extremely disadvantageous. Finally, if the ram boring machine at the tip of the drill rod gets stuck in the earth bore due to malfunctioning, there is no other alternative but to dig the ground right up.